Many combustion processes produce flue gases contaminated with carbon dioxide that contribute to global warming and environmental damage. Such gas streams are dilute, low-pressure, and difficult to treat; therefore, this gas cannot be economically transported to places where the carbon dioxide could be used.
To make transportation more feasible, the carbon dioxide can be separated and enriched to produce 90-100% pure carbon dioxide. While this concentrated carbon dioxide can be easily transported, the separation processes required to concentrate the carbon dioxide remain expensive. As such, there remains a need for better treatment options.
Gas separation by means of membranes is a well-established technology. In an industrial setting, a total pressure difference is usually applied between the feed and permeate sides, typically by compressing the feed stream or maintaining the permeate side of the membrane under partial vacuum.
It is known in the literature that a driving force for transmembrane permeation may be supplied by passing a sweep gas across the permeate side of the membranes, thereby lowering the partial pressure of a desired permeant on that side to a level below its partial pressure on the feed side. In this case, the total pressure on both sides of the membrane may be the same, the total pressure on the permeate side may be higher than on the feed side, or there may be additional driving force provided by keeping the total feed pressure higher than the total permeate pressure.
Using a sweep gas has most commonly been proposed in connection with air separation to make nitrogen or oxygen-enriched air, or with dehydration. Examples of patents that teach the use of a sweep gas on the permeate side to facilitate air separation include U.S. Pat. Nos. 5,240,471; 5,500,036; and 6,478,852. Examples of patents that teach the use of a sweep gas in a dehydration process include U.S Pat. Nos. 4,931,070, and 5,641,337.
Configuring the flow path within the membrane module so that the feed gas and sweep stream flow, as far as possible, countercurrent to each other is also known, and taught, for example in U.S. Pat. Nos. 5,681,433 and 5,843,209.
The use of a process including a membrane separation step operated in sweep mode for treating flue gas to remove carbon dioxide is taught in co-owned and copending patent application Ser. No. 12/734,941, filed Jun. 2, 2010.
Despite the innovations described above, the problem of capturing and sequestering carbon dioxide, so as to prevent its release to the atmosphere, or at least to delay it for many years, remains very difficult to solve in an energy- and cost-efficient manner. Solutions that have been proposed often rely on recovering the carbon dioxide in essentially pure form, such as by subjecting a stream containing the carbon dioxide to cryogenic distillation or amine sorption, followed by liquefaction. Although these solutions are potentially useful for some applications, there remains a need for a relatively simple, low-energy solution for treating carbon dioxide streams that avoids the requirement to create a high-purity stream.